a) Field of the Invention
This invention relates to a lockup piston with a damper, which is suitable for use in a torque converter.
b) Description of the Related Art
To facilitate the understanding of the present invention, reference is first had to FIGS. 7 and 8 of the accompanying drawings, in which FIG. 7 illustrates the basic construction of a torque converter equipped with a lockup clutch and FIG. 8 is an enlarged view of a conventional lockup piston. In these drawings, there are shown a torque converter main body 10, a connection portion 1 to an engine output shaft, a pump 2, a turbine 3, a stator 4, a turbine hub 5, a central axis 6, a power input casing 21, a core plate 22, retainer plates 23 fixed at crimped portions 26 thereof on the core plate 22, and tabs 24 each located on a radially inner side of a corresponding spring 32 which is in turn arranged on a radially inner side of an outer peripheral flange 28 of the core plate 22. Each of these tabs 24 has been formed by radially slitting an outer peripheral portion of its corresponding retainer plate 23 at two locations apart from each other to divide the outer peripheral portion into a central part and side parts, with which the central part is flanked, and then bending the central part in a direction toward the turbine 3. The core plate 22 and the retainer plates 23, in combination, make up a piston of the lockup clutch.
Designated at numeral 25 are spring end face support members. Each of these spring end support members 25 has been formed by bending one of the side parts of the outer peripheral portion of the corresponding retainer plate 23 into a turned square U-shape with the open face of the U rendered broader and directed toward the turbine 3. There are also shown a friction member 27 of the clutch, and a driven plate 31.
Since the technology of torque converters is well known, its detailed description is omitted herein. It is however to be noted that, when an input shaft and an output shaft become equal in rpm, occurrence of a slip cannot be avoided between fluid couplings.
To cope with this problem, a lockup clutch is arranged. When a hydraulic pressure is applied on a right side of the core plate 22 as viewed in FIG. 7 to bring the core plate 22 into contact under pressure with the input casing 21, the core plate 22 and the input casing 21 are connected directly with each other. Rotation of the core plate 22 is hence transmitted to the driven plate 31 via the springs 32. Since the driven plate 31 is fixed on the turbine 3, the input casing 21 and an output shaft are directly connected with each other via a damper which includes the springs 32. This has made it possible to avoid any slip.
As has been described above, a conventional lockup piston equipped with such a damper is arranged by crimping a retainer plate on which a spring is carried. Size and weight reductions are, however, under way with respect to automatic transmissions in which torque converters are arranged. To meet this trend, developments are also under way toward smaller and lighter torque converters. However, any attempt to reduce the size of each retainer plate of a lockup clutch, said retainer plate carrying a spring thereon, leads to a smaller crimped area. As a result, the force which binds the retainer plate with the core plate is reduced to a level lower than the conventional binding force. In general, this binding force consists in combination of resistance to shear force applied from a counterpart member, said resistance being hereinafter to be referred to as "anti-shear force", and resistance to force exerted in a direction to separate the retainer plate from the core plate. Of these, insufficient anti-shear force is particularly concerned. Described specifically, insufficient anti-shear force involves a potential problem in that the lockup piston may fail to exhibit sufficient function in transmitting a torque.